Conventionally, a method has been used in which a raw fuel such as coal, heavy oil, and OLIMULSION.TM. is partially oxidized into a gasified fuel. The gasified fuel is used as the fuel for gas turbines, etc. In recent years, various new fuels have been used as raw fuels.
For example, a technology for utilizing a natural tar called Olinoco tar as a fuel has been developed. Olinoco tar, which is obtained at the basin of the River Olinoco in Venezuela, South America, has a high viscosity in its normal state. Despite this high viscosity, Olinoco tar has a sufficient heating value, exhibiting properties of super-heavy oil. As shown in FIG. 2, Olinoco tar can be classified according to specific gravity (unit in the oil industry: API Baume degree); the lower the specific gravity, the higher the viscosity (kinematic viscosity). Therefore, Olinoco tar has the common characteristic in that its viscosity decreases with an increase in its temperature, though different specific gravity gives different temperature-viscosity characteristics.
Because Olinoco tar has such a high specific gravity at low temperatures, special methods are used for extraction and transportation of the tar. For the extraction, water is poured in Olinoco tar, and water and a surface active agent are added for getting an emulsion with a decrease in the high viscosity of the natural tar, which enables pumping-up and transportation of the emulsion by pipeline, etc. An emulsion obtained by adding water (about 30%) and a surface active agent to natural Olinoco tar (about 70%) has been commercialized as OLIMULSION.TM. (registered trade name of Bitumens Olinoco S.A.).
However, when a raw fuel is gasified and used as a gasified fuel, the gasification of the fuel is carried out at a high temperature of several hundred degrees centigrade. Therefore, when the raw fuel contains large quantities of water or it has water added to it prior or during gasification, the quantity of heat required for gasification is greatly increased, as a greater amount of heat is used to for heating or evaporating a large amount of water during gasification. Furthermore, when excess water is present in the fuel to be gasified, the size of the facility required to process the fuel into a gasified fuel increases. Additionally, the presence of excess water in the fuel to be gasified can lead to corrosion of the facility used, due to the generation of corrosive materials such as hydrogen chloride gas, which dissolve in condensed water.
Previously, natural Olinoco tar has been used as an emulsion when used as a fuel, such that the emulsion contains about 30% water. When this emulsion is then gasified in the presence of oxygen, the resulting gasified fuel contains about 14% water. Therefore, when this gas is cooled in order to remove impurities in the gas such as hydrogen sulfide, water is condensed and lost, resulting in a large heat loss. Moreover, since large quantities of water are contained in the gasified gas, sulfur content cannot be removed sufficiently by the dry gas refining method which utilizes iron oxide, etc.
Furthermore, when a raw fuel has a high water content, or has water added prior to gasification, the resultant gasified fuel has a high temperature above several hundred degrees centigrade and contains a large quantity of water vapor and sulfur compounds such as hydrogen sulfide. When desulfurization is carried out by bringing the gasified fuel into contact with an absorbent which is typically an amine, the gasified fuel must be cooled to a temperature close to ambient temperature to enhance the absorbing efficiency so that acidic gases such as hydrogen sulfide and carbon dioxide are absorbed. For this reason, high-temperature gasified fuel is cooled by using cooling water by means of a heat exchanger, and therefore the heat of condensation generated when water vapor turns to liquid water is lost.